Plate tectonics

Faulty thinking

The myth and reality of the Japanese earthquake

BACK in January, Japanese seismologists warned that the tectonic plates colliding beneath the Pacific Ocean off the north-east coast of Japan were poised to slip catastrophically. By their reckoning, there was a 99% chance of an earthquake of magnitude 8.0 occurring off the Miyagi coast, and a 90% chance of one off Ibaraki prefecture, within the next 30 years. They were surprised only by the sheer size of the magnitude 9.0 monster that was unleashed when the plates at last let go on March 11th.

It seems that, on occasions, the rupture along this particular type of “reverse fault” (where the upper part of one side of the fault is thrust over the foot of the other) can jump across gaps and other boundaries along the fissure, linking up with other parts of the fault to extend the breach alarmingly. The traditional fault-segmentation model used in seismology does not allow for this. But on that fateful Friday, when the North American plate slid over the Pacific plate along a subduction zone running 130km (80 miles) off the Pacific coast of northern Japan, the shock leapt from the first segment to a second and on to a third, extending the fault zone some 400km and increasing its intensity more than 30-fold. With all the action taking place only 24km down, the seabed was thrust violently upwards, triggering huge waves.

Could such a chain of seismic events happen elsewhere in Japan? Many in Tokyo fear that earthquakes may be creeping closer. The Japanese media have drawn attention to a quake of magnitude 6.6 on the far side of the country, between Nagano and Niigata prefectures, and to a quake of magnitude 6.1 in Shizuoka prefecture, both within days of the main quake. But these fairly common events occurred on entirely different tectonic plates. It is hard to imagine how faults on one continental plate might communicate with those on another that is hundreds of kilometres away.

No doubt Tokyo will be sideswiped one day. The most likely spawning ground for that earthquake will be 100km or more to the south-west, where the Philippine plate dives under the Eurasian plate, creating a continuous sequence of shudders. This could feasibly cause a megaquake of the kind the north-east has just suffered. But the evidence remains largely against it.